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How to paint metal surfaces

Author: NJR Steel
Article Views:20520

​Whether you are painting palisade fencing, IBR sheeting or anything else you have to follow the correct steps to ensure your coating lasts. Before specifying a coating for any metal surface, make a thorough assessment of the application. When selecting a coating for a project, three of the most important considerations are:

  1. The surface - What is the surface that needs to be coated? The most common paint-friendly metals are ferrous metals, galvanised metals and aluminium. Make sure the coating you specify will inhibit corrosion and adhere to the given surface.
  2. The environment - Under what conditions will the coating have to perform? Will it be indoors or outdoors, perhaps near the ocean? Such factors will influence your choice of coating.
  3. The surface preparation - Proper surface preparation is vital to the success of any paint job. It is especially important when selecting coatings for meal because of the vulnerability of the surface. Detailed guidelines on how to prepare metal are available from our experts.

Ferrous metals

Ferrous metals are those that either contain, or are derived from, iron, and are used in making castings, fabricated sheet steel, and wrought iron. With the exception of stainless steel, all of these metals will rust, which can eat away at the metal as well as spoil its appearance and undermine applied coatings. Rusting can start almost immediately when unprotected ferrous metal is exposed to rain, snow, dew or moisture in any form.

The overall objective is to stop any rusting that has begun; and to keep moisture and air from interfacing with the metal after painting. Accordingly, ferrous metals call for very thorough and meticulous surface preparation. Anything less may seriously compromise the appearance and durability of the finished paint job, not to mention the integrity of the metal itself.

Preparing the Surface

The first consideration when preparing a ferrous metal surface for painting in a noncorrosive atmosphere is to remove any loose rust that is present, as well as any peeling paint.

On smaller jobs, use a chisel-style scraper and a hand-held wire brush. Use the scraper to take off heavy rust and loose paint, then follow up by wire-brushing the surface to remove as much of the rust residue as possible. It is not necessary to remove every bit of rust, and take the surface down to bare metal, but rather to remove as much as these methods will allow.

On larger jobs, power wire-brushing or disk sanding with aluminum oxide paper is effective. Whether you use hand tools or power equipment to remove rust, be sure to wear personal protective equipment, including eye protection and a good dust mask.

After wire brushing, the surface will be covered with small particles of loose rust and dust, which should be removed before any coating is applied. Brush loose particles off with a soft bristle brush then scrub the surface with a detergent-and-water solution, followed by a thorough rinsing with clean water.

Surface preparation should not be omitted just because an iron or steel surface is new. New ferrous metal often has mill oil on it or small amounts of rust not readily visible. Not removing these before applying a coating could result in premature failure of the paint job.

Timing of priming is critical

timing is vital because rust can begin to re-form on iron or steel if the surface is exposed for as little as a day or two. If that happens, the surface will have to be prepared again.

When painting ferrous metal, it is important to apply a top quality metal primer because it must perform two vital functions: provide the bond between the topcoat and the metal, and inhibit corrosion. This is a point you should not compromise.

Utilise a quality exterior rust-inhibitive primer, apply the primer at the recommended spread rate in order to achieve adequate film thickness, which directly impacts corrosion resistance. Consider applying a second coat of primer for maximum corrosion resistance. Do not thin down the primer before application unless recommended by the manufacturer.

Top quality acrylic latex corrosion-inhibitive primers work well in applications where the metal is not exposed to heavily corrosive atmospheres, such as acidic or salt air. Unlike oil-based or solvent-based primers, they can be applied immediately after cleaning the surface, even if it is still slightly wet. Zinc rich and zinc chromate epoxy and alkyd primers are more appropriate for more highly corrosive settings.

Selecting the right topcoat is also important when painting ferrous metals. A high quality acrylic latex paint is generally a good choice because it can last as much as two to four times longer than conventional alkyd paints without serious cracking or fading, has a much quicker drying time, and is easier to handle and clean up.

Galvanized metal

Galvanized metal is iron or steel that has a thin coating of zinc on it to help pre-vent rusting. It is commonly used for gutters, downspouts and flashing.

If the galvanized surface is new or unweathered, wash and thoroughly rinse it be-fore painting. This step is necessary to clean off any zinc chromate or residual oil left from the galvanizing process, which otherwise can interfere with adhesion of the paint.

Apply a quality exterior acrylic latex corrosion-inhibitive primer for best perform-ance. However, if there are no signs of rust, a top quality exterior 100% acrylic latex paint can be applied to new galvanized metal without a primer.

Oil, alkyd and vinyl latex paints, however, should never be applied directly to bare galvanized metal without first applying a corrosion-inhibitive primer. Without an appropriate primer, these coatings can lose their adhesion to this substrate, sometimes in less than a year.

If the galvanized surface is weathered and unpainted, clean and rinse off any dirt and remove any rust with a nonmetallic scouring pad. Then apply a corrosion-inhibitive metal primer and top quality exterior 100% acrylic latex paint.

If it is previously painted, carefully remove any rust that is present and all loose and peeling paint with a wire brush. Avoid cutting through the layer of zinc galvanizing. Next, wash the surface with a detergent-and-water solution, rinse it thoroughly, and apply a corrosion-inhibitive metal primer followed by a 100% exterior acrylic latex paint.

How many coats?

When it comes to the number of coats of primer and paint to apply to metal, the cardinal rule is to follow the recommendations of the coating manufacturer. In general, however, one coat of primer is normally sufficient, although application of a second coat insures complete coverage and maximum protection in demanding situations.

Two coats of a topcoat are generally recommended rather than a single coat for two reasons: increased overall thickness of the coating for better ultimate durability, and elimination of the possibility of any pinholes extending through the coating.

In addition, always apply primer and paint to metal in thick coats for optimum durability and rust resistance. That’s because the thicker the coat, the less chance of moisture penetrating the paint and reaching the substrate. Brushing is fine for small applications, while rolling and airless spraying are better for covering large areas.

Direct-to-metal coatings

Another option when painting metal is Direct-To-Metal (DTM) coatings. These coatings are designed for direct application to metals without the use of a primer. They function as both primer and topcoat in one product.

DTM coatings can be used for a variety of applications, are available in semigloss or gloss formulations, and are now offered in high performance latex products. To insure proper protection of the metal, two thick coats are usually recommended.

Regardless of whether you use a DTM coating or a primer-topcoat system, it is not difficult to get years of reliable service from a metal paint job. The keys are taking the time to properly prepare the surface, promptly priming the surface, and applying thick coats of the right type of coating.

Source: Paint Quality Institute

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